Why object from Git.tagList().call() is not a tag?

I’m trying Jgit and I want to retrieve all tags of a cloned repository and I want to sort them by Date. That’s why I’m trying the following:

String path = "path/to/my/git/repo/.git";
Repository repository = builder.setGitDir(new File(path)).readEnvironment().findGitDir().build();
try (RevWalk walk = new RevWalk(repository)) {
            List<Ref> taglist = new Git(repository).tagList().call();

            Collections.sort(taglist, new Comparator<Ref>() {
                public int compare(Ref o1, Ref o2) {
                    Date d1 = null;
                    Date d2 = null;
                    try {
                        d1 = walk.parseTag(o1.getObjectId()).getTaggerIdent().getWhen();
                        d2 = walk.parseTag(o2.getObjectId()).getTaggerIdent().getWhen();
                    } catch (IOException e) {
                        e.printStackTrace(); 
                    }
                    return d1.compareTo(d2);
                }
            });
        }catch (GitAPIException e){
            e.printStackTrace();
        }
    }

The problem is that some of Ref returned by this.taglist = new Git(repository).tagList().call() are not Tag: in fact an exception is raised by parseTag() and its message is: org.eclipse.jgit.errors.IncorrectObjectTypeException: Object 3ea2b388b5ef02622312fe08e6935a5f9c655e00 is not a tag.

Can someone explain me why this happen and how can i solve the problem?
Thank you all.

Answer

As I said in the comment, i solved changing the walk.parseTag(o1.getObjectId()).getTaggerIdent().getWhen(); into walk.parseCommit(o1.getObjectId()).getAuthorIdent().getWhen(); and same for o2. Hope this will help others.